The present invention relates to a purification method and a purification apparatus capable of effectively purifying a contaminated object containing halogenated organic compounds. More particularly, the invention relates to a purification method and a purification apparatus capable of purifying a contaminated object containing chlorinated organic compounds, such as soil, sediment, sludge, or water, e.g., interstitial water of sludge or groundwater, by a chemical reaction or a reductive dehalogenation reaction comprising a combination of a chemical reaction and a biological reaction, to degrade the halogenated organic compounds efficiently.
The entire disclosure of an international application PCT/JP/98/00363, filed with the Japanese Patent Office on Jan. 29, 1998, entitled xe2x80x9cA Method for Purifying Object Contaminated with Halogenated Organic Compounds,xe2x80x9d is cited in the present patent application.
In recent years, there have been one report after another on contamination of soil and groundwater with halogenated organic compounds, such as tetrachloroethylene, trichloroethylene, 1,1,1-trichloroethane, and dichloroethylene, which are widely used as degreasing agents for metal components of electronic equipment, and cleaning agents for dry cleaning. Recently, in particular, attention has been attracted to contamination with dioxins discharged from incineration facilities, PCB, etc. These halogenated organic compounds are not easily degraded in the natural world, and sparingly soluble in water. Thus, they tend to be accumulated in the soil and penetrated into groundwater in the contaminated area. The halogenated organic compounds are also known to cause hepatic damage and have carcinogenicity. It is desired, therefore, that halogenated organic compounds, such as chlorinated organic compounds, contained in the soil, etc. be degraded and rendered harmless.
Recently, bioremediation has attracted attention as a method of purifying soil, groundwater, etc. contaminated with halogenated organic compounds. The bioremediation method is highly cost-effective and has high safety. Bioremediation, however, has the problems that treatment requires a long time, and the types and concentrations of substances which can be degraded are restricted, as will be described below.
Aerobic degradation of trichloroethylene by methane assimilating organisms, toluene or phenol degrading organisms, ammonia oxidizing bacteria, or alkene assimilating organisms is known as an example of bioremediation. However, this method has the following drawbacks: 1) Degradation reaction is unstable. 2) The scope of substances to be degraded is very narrow. 3) It has no degrading action on highly chlorinated substances such as tetrachloroethylene and carbon tetrachloride.
Many anaerobic microorganisms, on the other hand, can degrade highly chlorinated organic compounds, such as tetrachloroethylene, trichloroethylene, and carbon tetrachloride, and have a broad range of applications. However, they are defective, for example, in that 1) growth of the microorganisms is very slow, and 2) strongly toxic intermediary metabolites are formed and accumulated during the anaerobic degradation process (Uchiyama HIROO and Yagi OSAMI, Bioscience and Industry, 1994, Vol. 52, No. 11, 879-884).
As a technique for decomposing halogenated organic compounds by a chemical reaction, reductive treatment of chlorinated organic compounds with metallic iron has been reported (Yazaki TETSUO, Treatment of Organochlorine Compound Contaminated Groundwaterxe2x80x94Technology for Treatment at Low Temperatures with Metallic Iron Deposited Activated Carbon, xe2x80x9cPPMxe2x80x9d, 1995, Vol. 26, No. 5, 64-70). Thus, the inventor of the present invention attempted to conduct dechlorination experiments by adding metallic iron into the soil in the absence of a carbon source for microorganisms. However, no dechlorination reaction was observed under conditions under which no microorganisms were cultured, particularly when a reductive atmosphere and neutral conditions were not maintained. Nor was any dechlorination reaction noted when an iron salt, such as FeCl2, FeCl3 or FeSO4, was added instead of metallic iron.
Furthermore, there is a report of a method for treating halogenated organic compounds to become harmless, which comprises injecting metallic iron and high pressure air into contaminated soil to react the halogenated organic compounds with an iron powder to make it inorganic (Japanese Unexamined Patent Publication No. 1996-257570). This method involves a problem about air injection facilities, and a risk of vaporization of the halogenated organic compounds. The method is also impractical, since the use of high pressure air causes the problem of cost.
There is also a report of a method for removing organochlorine compounds, which have contaminated soil or groundwater, by combining a natural substance having a dehalogenation catalytic action and bioremediation (xe2x80x9cNikkei Biotechxe2x80x9d (Nikkei BP), published Oct. 7, 1996, No. 361, 14-15). However, this report is silent on concrete natural substances and microorganisms.
U.S. Pat. No. 5,411,664 describes a method for degradation of halogenated organic compounds by addition of a fibrous organic material and polyvalent metal particles, e.g., iron, to a contaminated object. However, this U.S. patent does not describe a reducing agent, such as reduced iron, cast iron, an alloy, or a water-soluble reducing agent. Nor does this patent describe that the contaminated object is held in a reducing atmosphere after addition of a reducing agent.
In the laboratory, it is easy to mix a reducing agent, a nutrient source, and a contaminated object uniformly. To purify a contaminated object, such as soil, actually in situ, on the other hand, large amounts of a reducing agent and a nutrient source are mixed, thus requiring construction work. Moreover, uniform mixing is not necessarily easy. In addition, the conditions during kneading may affect the degradation ratio of halogenated organic compounds. To purify a contaminated object of a volume of 1 m3 or more, particularly, a volume of 10 m3 or more, an ingenious idea is needed for the kneading method.
Besides, to purify a contaminated object, such as soil, in situ, it has been common practice to perform a small amount of pumping downstream from a contaminated aquifer, apply purification treatment for the contaminated object to the pumped groundwater, then dissolve a nutrient source for microorganisms, which degrade the contaminant, in the treated groundwater, and inject the groundwater again into an upstream unsaturated soil or aquifer. With this method, however, the pumped contaminated water cannot be reinjected unless it is purified below the concentration complying with the ban on the permeation of effluent into the ground. Thus, it is necessary to install purification facilities on the ground, posing the problem of an increased cost. Also, the injected normal water brings about a phenomenon of a detour formed in the flow of the contaminated groundwater. This phenomenon poses the problem that the injected nutrient source does not thoroughly mix with the groundwater.
Therefore, the object of the present invention is to provide a contaminated object purification method and purification apparatus which can efficiently and easily purify a contaminated object containing halogenated organic compounds. Particularly, its object is to provide a purification method and a purification apparatus which can efficiently and easily purify a contaminated object containing halogenated organic compounds, especially soil and groundwater, in situ.
According to one aspect of the present invention, water is circulated so as to pass through the contaminated object, for example, the contaminated object in the soil. According to another aspect of the present invention, circulation of water in this manner is not required.
According to an aspect of the present invention, there is provided a purification method for purifying a contaminated object containing halogenated organic compounds, including a reduction step in which a reducing agent having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of from 300 mV to xe2x88x922400 mV reduces a nutrient solution containing a nutrient source for heterotrophic anaerobic microorganisms and water, and an introduction step in which the reduced nutrient solution is introduced into the contaminated object after the reduction step. Preferably, the introduction step is performed after, preferably immediately after, the reduction step.
The purification method according to this aspect of the present invention combines a chemical reaction and a biodegradation reaction, and can degrade the halogenated organic compounds to purify the contaminated object containing the halogenated organic compounds. By introducing the reduced nutrient solution into the contaminated object after, preferably immediately after, the reduction step, the reduced state of the nutrient solution can be maintained, and the activity of the microorganism can be enhanced to purify the contaminated object more efficiently. Here, xe2x80x9cimmediately after the reduction stepxe2x80x9d refers to xe2x80x9cafter a lapse of a sufficiently short time to maintain the reduced state of the nutrient solution.xe2x80x9d
In the introduction step, a well, an underground wall, a permeation gutter, a trench, or an indentation is preferably used.
The reduction step preferably includes a contact step of bringing the nutrient solution into contact with the reducing agent which is in a solid state and is insoluble or sparingly soluble in water.
Alternatively, the reduction step preferably includes a mixing step of mixing the nutrient solution with an aqueous solution containing the reducing agent which is water soluble.
In the introduction step, the reduced nutrient solution is preferably introduced into the contaminated object via a deep indentation provided in the ground surface, for example, a well, or an underground wall. In the ground with high permeability, however, the reduced nutrient solution may be introduced via a shallow indentation.
It is preferred to further have a step of circulating groundwater containing the halogenated organic compounds and provide the reduction step and the introduction step while circulating the groundwater.
Herein, the halogenated organic compounds refer to fluorinated organic compounds, chlorinated organic compounds, brominated organic compounds, or iodinated organic compounds. Particularly, the invention is targeted at, but not limited to, aliphatic compounds such as tetrachloroethylene, trichloroethylene, 1,1,1-trichloroethane or dichloroethylene, and aromatic compounds such as pentachlorophenol, which present problems as contamination sources for groundwater and soil.
The contaminated object includes, for example, groundwater, soil, sediment, sludge, compost, manured organic substances, waste, and drainage.
The ratio between the nutrient source for the heterotrophic anaerobic microorganisms and water that are contained in the nutrient solution is not restricted. The nutrient source for the heterotrophic anaerobic microorganisms can be selected, as desired, according to the properties of the microorganisms in the contaminated object to be purified.
Preferred examples of the heterotrophic anaerobic microorganisms are methane-forming bacteria (e.g., the genus Methanosarcina, the genus Methanothrix, the genus Methanobacterium, the genus Methanobrevibacter), sulfate-reducing bacteria (e.g., the genus Desulfovibrio, the genus Desulfotomaculum, the genus Desulfobacterium, the genus Desulfobacter, the genus Desulfococcus), nitrate-reducing bacteria (e.g., the genus Bacillus, the genus Lactobacillus, the genus Aeromonas, the genus Streptococcus, the genus Micrococcus), acid-forming bacteria (e.g., the genus Clostridium, the genus Acetivibrio, the genus Baceroides, the genus Ruminococcus), and facultative anaerobic bacteria (e.g., the genus Bacillus, the genus Lactobacillus, the genus Aeromonas, the genus Streptococcus, the genus Micrococcus). Particularly, the genera Bacillus, Pseudomonas, Aeromonas, Streptococcus and Micrococcus are preferred, because they have oxide form nitrogen reducing activity.
Examples of the nutrient source which can be used preferably when the heterotrophic anaerobic microorganisms are methane-forming bacteria are shown in Tables 1 and 2 below.
The mineral 1 solution refers to a solution containing 6 g of K2HPO4 in 1 liter of distilled water.
The mineral 2 solution refers to a solution containing 6 g of KH2PO4, 6 g of (NH4)2SO4, 12 g of NaCl, 2.6 g of MgSO4.7H2O, and 0.16 g of CaCl2.2H2O in 1 liter of distilled water.
The trace mineral solution refers to a solution containing 1.5 g of nitrilotriacetic acid, 3.0 g of MgSO4.7H2O, 0.5 g of MnSO4 2H2O, 1.0 g of NaCl, 0.1 g of FeSO4.7H2O, 0.1 g of CoSO4 or CoCl2, 0.1 g of CaCl2.2H2O, 0.1 g of ZnSO4, 0.01 g of CuSO4.5H2O, 0.01 g of AlK(SO4)2, 0.01 g of H3BO3, and 0.01 g of Na2MoO4.2H2O in 1 liter of distilled water. First, nitrilotriacetic acid is dissolved while being adjusted to pH 6.5 with KOH, and then the other minerals are added. Finally, the solution is adjusted to pH 7.0 with KOH.
The trace vitamin solution refers to a solution containing 2 mg of biotin, 2 mg of folic acid, 10 mg of pyridoxine hydrochloride, 5 mg of thiamine hydrochloride, 5 mg of riboflavin, 5 mg of nicotinic acid, 5 mg of calcium DL-pantothenate, 0.1 mg of vitamin B12, 5 mg of p-aminobenzoic acid, and 5 mg of lipoic acid in 1 liter of distilled water.
An example of the nutrient source which can be used preferably when the heterotrophic anaerobic microorganisms are sulfate-reducing bacteria is shown in Table 3 below.
An example of the nutrient source which can be used preferably when the heterotrophic anaerobic microorganisms are nitrate-reducing bacteria is shown in Table 4 below.
The use of organic carbon and a culture medium, which contains 20 to 50% by weight of organic carbon, preferably 20 to 30% by weight of oxide form nitrogen, as the nutrient source is preferred, because the microorganism population involved in the aforementioned chemical reaction and biological reaction can be changed to suppress blackening of the soil due to iron sulfide, etc., occurrence of a methane gas, and generation of foul-smelling gases such as mercaptan. Furthermore, a nitrogen gas is generated, producing the advantage that the resulting hydrogen gas is diluted. The oxide form nitrogen is preferably in the form of a nitrate. A preferred example of the nitrate is an alkali metal salt of nitric acid, an alkaline earth metal salt of nitric acid, iron nitrate, titanium nitrate, manganese nitrate, aluminum nitrate, or magnesium nitrate. Particularly, sodium nitrate, potassium nitrate or calcium nitrate can be used preferably. The organic carbon is preferably a water soluble organic carbon source. Preferred examples of the organic carbon source are sugars, organic acids or their derivatives, lower alcohols, molasses waste liquor, fermentation waste liquor, and mixtures of them.
In connection with the reducing agent used in the present invention, its standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of higher than 300 mV is not preferred, because a sufficient reducing power is not obtained. The standard electrode potential of less than xe2x88x922400 mV is not preferred, either, because the reducing power is so strong that a hydrogen gas may be generated, posing a danger. The standard electrode potential (Exc2x0) relative to a standard hydrogen electrode at 25xc2x0 C. is shown in Table 5.
As the reducing agent, a reducing agent which is in a solid state and is insoluble or sparingly soluble in water, or a water soluble reducing agent can be used preferably.
As the reducing agent in solid state, there can be preferably cited at least one reducing agent selected from the group consisting of reduced iron, cast iron, iron-silicon alloy, titanium alloy, zinc alloy, manganese alloy, aluminum alloy, magnesium alloy, calcium alloy, titanium-silicon alloy, titanium-aluminum alloy, zinc-aluminum alloy, manganese-magnesium alloy, aluminum-zinc-calcium alloy, aluminum-tin alloy, aluminum-silicon alloy, and calcium-silicon alloy.
When reduced iron is used as the solid state reducing agent, adsorption of the halogenated organic compounds to the surface of the reduced iron occurs. Simultaneously, polarization into an anode and a cathode takes place on the surface of the reduced iron because of the difference in conditions between the metal side and the environment side to flow an electric current. Accordingly, iron solves out as iron ions on the anode as shown in the following formula 1, and electrons flow into the cathode, causing a reduction reaction:
Fexe2x86x92Fe2++2exe2x88x92xe2x80x83xe2x80x83Formula 1
When cast iron is used as the reducing agent, graphite contained in the cast iron adheres to the surface. This graphite acts as a cathode, while the iron acts as an anode. When the alloy is used as the reducing agent, polarization into an anode and a cathode occurs according to the standard electrode potentials of the metallic elements making up the alloy. In the case of the alloy, a reducing atmosphere can be maintained more easily, and the potential difference from the halogenated organic compounds becomes greater. Thus, a dehalogenation reaction is promoted.
When the reducing agent in a solid state and insoluble or sparingly soluble in water is used as the reducing agent, it is preferred for the reducing step to include a contact step of contacting the nutrient solution with the reducing agent. Inclusion of the contact step permits a thorough reaction between the reducing agent insoluble or sparingly soluble in water and the nutrient solution, leading to reduction of the nutrient solution.
As the solid state reducing agent, a powder having a particle size of 500 xcexcm or less can be cited preferably.
As the water soluble reducing agent, there can be preferably named an organic acid or its derivative, hypophosphorous acid or its derivative, or a salt of an organic acid or hypophosphorous acid with iron, titanium, zinc, manganese, aluminum or magnesium, or a sulfide salt.
As the organic acid, a carboxylic acid, a sulfonic acid, a phenolic acid, or a derivative thereof can be cited preferably. Preferred examples of the carboxylic acid are monocarboxylic acids, dicarboxylic acids, tricarboxylic acids and tetracarboxylic acids having 1 to 20 carbon atoms and optionally substituted by hydroxyl groups. Concretely, acetic acid, citric acid and terephthalic acid are preferred, and aliphatic tricarboxylic acids having 2 to 10 carbon atoms, such as citric acid, are particularly preferred. As the derivative of the phenolic acid, a polyhydroxyaryl can be named preferably. As the polyhydroxyaryl, 1,2,3-trihydroxybenzene and 1,4-dihydroxybenzene are preferred. As the derivatives of the organic acid, salts, esters, amides, and acid anhydrides can be preferably cited.
As the derivatives of hypophosphorous acid, salts and esters can be cited preferably, and salts are particularly preferred.
The use of the water soluble reducing agent, compared with the use of the reducing agent in solid state, dramatically increases the efficiency of contact with the halogenated organic compounds, thereby promoting the dehalogenation reaction. Also, the water soluble reducing agent permeates the soil, etc., and thus can be injected using a conduit, such as a well, or an underground wall. Furthermore, the reduced state can be easily recovered by adding the reducing agent according to the reduced state during the purification operation.
When the water soluble reducing agent is used, it is preferred for the reduction step to include a mixing step of mixing the nutrient solution with an aqueous solution containing the reducing agent. By including the mixing step, the water soluble reducing agent and the nutrient solution can be fully reacted, and the nutrient solution can be reduced.
In the introduction step, the reduced nutrient solution is preferably introduced into the contaminated object via a deep indentation provided in the ground surface. The deep indentation is a concept including a conduit inserted into the ground and set in place, a well, an underground wall, or the like. If the deep indentation is a conduit or well, the conduit or well preferably includes in at least a part thereof a strainer portion comprising many through-holes. The underground wall refers to a deep groove, and typically refers to one formed with a highly water permeable filler, such as sand or gravel, fitted into the groove so that the groove will not collapse. The underground wall has excellent air permeability and water permeability.
In the ground with satisfactory water permeability, the reduced nutrient solution may be introduced via a permeation gutter or a shallow indentation such as a trench. As the ground with satisfactory water permeability, a stratum comprising sand, gravels or pumice, for example, can be cited. The depth of the trench depends on how easily the ground collapses. Near a place where vehicles, etc. pass, for example, the trench may be about 20 to 30 cm deep, or even deeper. At a place where vehicles, etc. do not pass and the ground is stable, the depth may be about 1 m or more. The wall of the trench may be reinforced to prevent collapse, or need not be reinforced.
According to the present invention, there is also provided a purification apparatus for purifying a contaminated object containing halogenated organic compounds, comprising reduction means by which a reducing agent having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of from 300 mV to xe2x88x922400 mV reduces a nutrient solution containing a nutrient source for heterotrophic anaerobic microorganisms and water, and introduction means by which the reduced nutrient solution is introduced into the contaminated object via an introduction portion for introducing the reduced nutrient solution into the contaminated object.
As the reducing agent and the nutrient solution, the aforementioned reducing agent and the nutrient solution can be used preferably.
The reduction means preferably includes a contact device for bringing the nutrient solution and the reducing agent into contact with each other, if the reducing agent is in a solid state and is insoluble or sparingly soluble in water. If the reducing agent is water soluble, on the other hand, the reduction means preferably includes a mixing device for mixing the nutrient solution and an aqueous solution containing the reducing agent. In either case, the nutrient solution can be fully reduced with the reducing agent.
The introduction portion is preferably a deep indentation formed in the ground surface. The deep indentation is a concept including a conduit inserted into the ground and set in place, a well, an underground wall, or the like. In the ground with satisfactory water permeability, the reduced nutrient solution may be introduced via a permeation gutter or a shallow indentation such as a trench.
The reduction means preferably includes an underground wall filled with a water permeable filler, and the reducing agent is preferably used as at least a part of the filler.
The reducing agent may be in a particulate form, such as sand or gravel, or need not be particles. For example, a metal powder, such as an iron powder, having a particle size comparable to that of sand or gravel may be used as the filler. Alternatively, the filler may contain sand or gravel, and a metal powder such as an iron powder. Alternatively, the filler may contain a filler such as sand or gravel, and a reducing agent insoluble or sparingly soluble in water. In this case, the filler such as sand or gravel is a main component of the underground wall which retains air permeability and water permeability, and the reducing agent is contained in such an amount as not to impair the air permeability and water permeability. For example, 50% by weight or less of the reducing agent is contained, or 30% by weight or less of the reducing agent may be contained. For example, 20% by weight or less of the reducing agent may be contained. The underground wall may have an upper portion sealed or not sealed. Alternatively, as will be mentioned later, it may be a shallow indentation, rather than a deep indentation.
The introduction means preferably has a pump.
Moreover, the purification apparatus preferably has pumping means for introducing the contaminated object into the introduction portion for circulation treatment of the contaminated object. In this case, the contaminated object is mixed in the reduction means a plurality of times, and thereby dehalogenated more fully.
According to another aspect of the present invention, there is provided a purification method for purifying a contaminated object containing halogenated organic compounds, including a water reduction step of reducing water with a reducing agent having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of 300 mV to xe2x88x922400 mV, a contact step of bringing the water reduced in the reduction step into contact with a nutrient source for heterotrophic anaerobic microorganisms to obtain a mixture containing the nutrient source, and an addition step of adding the mixture obtained in the contact step to the contaminated object.
According to an embodiment, there is provided a purification method for purifying a contaminated object containing halogenated organic compounds, including a water reduction step of reducing water with a reducing agent having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of from 300 mV to xe2x88x922400 mV, a mixing step of mixing the water reduced in the reduction step with a nutrient source for heterotrophic anaerobic microorganisms, and an addition step of adding a mixture obtained in the mixing step to the contaminated object.
The nutrient source may be a liquid, namely, water containing a nutrient source, or may be a solid. In the case of the solid, there can be used, for example, a solid organic material such as compost, manure, excess sludge, sludge with a high organic matter content, or organic waste.
An embodiment in which the nutrient source is a nutrient solution will be mainly described. In the present invention, it is preferred to include a nutrient solution reduction step of adding water reduced in the reduction step to a nutrient solution containing a nutrient source for heterotrophic anaerobic microorganisms to reduce the nutrient solution, and a step of adding the nutrient, solution reduced in the nutrient solution reduction step to the contaminated object. However, reduced water and a nutrient source in the form of a solid, such as powder, a sol or a gel may be mixed or contacted with each other. As the solid nutrient source, an organic waste such as compost, bean curd refuse, or beer cake, or humus soil can be used as such, or any of them which has been compacted into a solid or semisolid form can be used.
When the nutrient source is a solid, water, or water which has already been reduced by passage through a reducing agent may be contacted with the solid nutrient source to dissolve the nutrient source into water. For example, water may be passed through a tank holding the solid nutrient source, or a packed column packed with the solid nutrient source.
As the reducing agent and the nutrient solution, the above-mentioned reducing agent and nutrient solution can be used preferably.
It is preferred that a well, an underground wall, a permeation gutter, a trench or an indentation is used in the water reduction step, the contact step, or the addition step.
According to another aspect of the present invention, there is provided a purification method for purifying a contaminated object containing halogenated organic compounds, including a circulation step of circulating water so as to pass through the contaminated object, and a reduction step of reducing the circulating water with a reducing agent having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of 300 mV to xe2x88x922400 mV.
Herein, the aforementioned xe2x80x9ccontaminated objectxe2x80x9d includes soil, groundwater, sludge, sediment, compost, manured organic substances, waste, drainage (each containing a contamination source), and all other contaminated objects contaminated with halogenated organic compounds. Particularly, this term includes soil, groundwater, sludge (each containing a contamination source), and all other contaminated objects contaminated with halogenated organic compounds.
The aforementioned xe2x80x9cwaterxe2x80x9d includes water containing a contamination source such as halogenated organic compounds, and water after reduction in the reduction step. Concretely, the term xe2x80x9cwaterxe2x80x9d includes not only groundwater, but also free water in sludge, soil, paddy field or bottom mud, and water as an aqueous solution supplied to the contaminated object from outside a system, such as a ground surface portion.
The purification method according to this aspect of the present invention involves circulating water, especially groundwater, and repeatedly performing the reduction step for the circulating water with the use of the reducing agent, whereby water, and the contaminated object in contact with water, especially sludge or solid, can be purified merely by a reducing action.
The reduction step is preferably performed in the soil. By performing the reduction step in the soil, there is no need to pump contaminated groundwater above the ground, permitting purification at a low cost. Secondary contamination, which may occur as a result of pumping of the contaminated groundwater, can also be prevented.
The circulation step preferably has a step of taking in water in the soil, and a step of discharging water into the soil. This circulation step can gradually purify contaminated groundwater without causing a rapid change in the composition of groundwater.
The reducing agent is preferably in a solid state, and insoluble or sparingly soluble in water. The use of such reducing agent makes it possible to reduce circulated contaminated water repeatedly, without inducing a marked decrease in the reducing agent. Thus, contaminated water can be purified in the soil at a low cost. During this circulation, moreover, the contaminated object is cleaned physically, and can be purified thereby.
Alternatively, the reducing agent is preferably a water soluble reducing agent. When the water soluble reducing agent is used, permeation of the reducing agent through the soil is expected.
In the purification method of the present invention, it is preferred to include a reducing agent addition step of adding the reducing agent. Furthermore, the reducing agent addition step is preferably performed on the ground surface, because handling is easy on the ground surface. Alternatively, this step may be performed in the soil. For example, the reducing agent may be mixed with the filler of the underground wall, as has been stated earlier.
It is further preferred to have a nutrient source contact step of bringing a nutrient source for heterotrophic anaerobic microorganisms into contact with water before being circulated.
There may be provided a nutrient source contact step of further bringing a nutrient source for heterotrophic anaerobic microorganisms into contact with water being circulated.
It is preferred in handling that the nutrient source is added as a nutrient solution containing a nutrient source. An embodiment of adding the nutrient source as a nutrient solution will be mainly described. However, the nutrient source in the form of a solid, such as powder, a sol or a gel may be added to water being circulated.
In an embodiment using the nutrient source in this manner, water, especially groundwater, is circulated, and the reducing agent and the nutrient solution are added to water being circulated. By so doing, a chemical reaction by the reducing agent, and a biodegradation reaction by anaerobic microorganisms activated by the nutrient solution are repeatedly performed. Thus, water, and the contaminated object in contact with water, especially, sludge or soil, can be efficiently purified.
In the addition step, which of the addition of the reducing agent and the addition of the nutrient solution may be performed first. What is important is that the nutrient solution is maintained in a reduced state when contacted with the contaminated object. By so maintaining the nutrient solution in the reduced state, the microbial activity can be enhanced, and the contaminated object can be purified with higher efficiency.
The proportion of the nutrient source for heterotrophic anaerobic microorganisms and water incorporated into the nutrient solution is not restricted. The nutrient source for heterotrophic anaerobic microorganisms can be selected, as desired, according to the properties of microorganisms in the contaminated object to be purified. As the heterotrophic anaerobic microorganisms, those stated above can be used.
In the above step of taking in water present in the soil, it is preferred that a well, an underground wall, a permeation gutter, a trench or an indentation is used.
It is also preferred that a well, an underground wall, a permeation gutter, a trench or an indentation is used in the step of discharging water into the soil.
The use of the water soluble reducing agent, compared with the use of the reducing agent in solid state, dramatically increases the efficiency of contact with the halogenated organic compounds, thereby promoting the dehalogenation reaction. Also, the water soluble reducing agent permeates the soil, etc., and thus can be injected using a conduit, such as a well, an underground wall, a permeation gutter, a trench, or a shallow indentation. Furthermore, the reduced state can be easily recovered by adding the reducing agent according to the reduced state during the purification operation.
When the water soluble reducing agent is used, moreover, it is preferred to mix the nutrient solution with an aqueous solution containing the reducing agent in the addition step. By so doing, the water soluble reducing agent and the nutrient solution can be fully reacted, and the nutrient solution can be maintained in a fully reduced state.
In the addition step, the reduced nutrient is preferably introduced into the contaminated object via a deep indentation provided in the ground surface. The deep indentation is a concept including a conduit inserted into the ground and set in place, a well, an underground wall, or the like. If the deep indentation is a conduit or well, the conduit or well preferably includes in at least a part thereof a strainer portion comprising many through-holes. The underground wall refers to a deep groove, and typically refers to one formed with a highly water permeable substance, such as sand or gravel, filled into the groove so that the groove will not collapse. Thus, the underground wall has excellent air permeability and water permeability. On this occasion, the reducing agent in solid state, may be used as a filler for the underground wall, as stated earlier. Alternatively, a permeation gutter, a trench, or a shallow indentation may be used during introduction of the nutrient solution.
According to another aspect of the present invention, there is provided a purification apparatus for purifying a contaminated object containing halogenated organic compounds, characterized by having a reducing agent having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of from 300 mV to xe2x88x922400 mV, a water intake portion located in the contaminated object or downstream from the contaminated object, and a water discharge portion located upstream from the water intake portion and located in the contaminated object or upstream from the contaminated object, and characterized in that water is circulated among the water intake portion, the reducing agent, and the water discharge portion to purify the contaminated object. For example, water flows through the water intake portion, the reducing agent, and the water discharge portion in this order. From the water discharge portion, water is discharged into the soil, flows under gravity or along the flow of groundwater, and is taken in again from the water intake portion. The water intake portion is preferably disposed in an aquifer.
The reducing agent preferably includes a reducing agent in a solid state and insoluble or sparingly soluble in water.
The reducing agent preferably includes a water soluble reducing agent.
The reducing agent is preferably located between the water intake portion and the water discharge portion. This is because groundwater is first taken in from the water intake portion, then transported to the reducing agent by a pump or the like, and then reduced with the reducing agent, whereafter the reduced groundwater is discharged from the water discharge portion. The discharged water can be moved to the water intake portion by gravity or the flow of groundwater.
The reducing agent is preferably held in a reducing agent tank. The reducing agent tank may be provided in the soil, or may be provided on the ground surface.
The water intake portion or the water discharge portion is preferably provided in a well, an underground wall, a permeation gutter, a trench or an indentation. It is preferred to have a pump in relation to circulation of water. For example, the pump may be disposed between the water intake portion and the water discharge portion. Alternatively, the pump may be disposed above the water intake portion.
It is preferred to further have a nutrient source for heterotrophic anaerobic microorganisms. It is preferred that water before being circulated is contacted with the nutrient source. It is preferred to circulate water among the water intake portion, the reducing agent, the nutrient source, and the water discharge portion, thereby purifying the contaminated object. For example, water flows through the water intake portion, the reducing agent, the nutrient source, and the water discharge portion in this order, or flows through the water intake portion, the nutrient source, the reducing agent, and the water discharge portion in this order, is discharged from the water discharge portion into the soil, flows by gravity or along the flow of groundwater, and is taken in again from the water intake portion.
It is preferred to further have a nutrient source tank holding a nutrient source.
According to another aspect of the present invention, there is provided a purification apparatus for purifying contaminated soil containing halogenated organic compounds, which includes an indentation formed in the ground surface above or upstream from the contaminated soil, a reducing agent disposed in the indentation and having a standard electrode potential, relative to a standard hydrogen electrode at 25xc2x0 C., of 300 mV to xe2x88x922400 mV, and a nutrient source for heterotrophic anaerobic microorganisms which is disposed above the reducing agent.
For example, a shallow indentation, such as a trench, may be dug in the ground surface, a reducing agent may be filled as a first layer into a lower part of the indentation, and a nutrient source for heterotrophic anaerobic microorganisms may be filled as a second layer into an upper part of the indentation. In this case, the indentation is provided above or upstream from the contaminated object containing the halogenated organic compounds in the soil. Groundwater or tap water may be sprinkled over the second layer. Alternatively, rainwater may be allowed to flow into the second layer, without performing the sprinkle. As a result, the sprinkled water or the rainwater passes through the nutrient source, turning into water containing the nutrient source, and then the water is reduced with the reducing agent. The reduced water seeps into the soil, reaching the contaminated object in the soil and purifying it.
The reducing agent is preferably a solid. The nutrient source is preferably a solid. As the solid nutrient source, it is preferred to use a solid organic material such as compost, manure, excess sludge, sediment with a high organic matter content, or organic waste.
The reducing agent and the nutrient source may be each arranged in a layer, and the layer comprising the nutrient source covers the layer comprising the reducing agent. Since the solid nutrient source covers the reducing agent, the reducing agent can be prevented from being consumed by reducing oxygen in the air.